The ongoing trends toward larger wafers, shrinking line widths, and ever thinner oxides are making tight in-line monitoring of wafer cleanliness and uniformity even more critical to semiconductor manufacturers. Contaminants can be any form of matter that causes unintentional changes in electrical properties of semiconductor devices. Some common contaminants include particles, atomic-ionic-molecular defects, and heavy metals.
The fabrication of complimentary metal oxide semiconductor (CMOS) devices involves numerous distinct manufacturing process steps. Device contamination, during any of these processes, poses a serious quality control problem and when severe, may necessitate that the devices be scrapped. To monitor contamination that occurs during the manufacturing process, manufacturers have developed tests that attempt to monitor contamination in the semiconductor device.
A measurement of electrically active contamination may be accomplished using a resistivity test. This is often done through the use of a tool having four probes that actually touch the top of the semiconductor wafer, where the tool measures the resistivity between the probes. These measurement probes themselves, however, tend to become contaminated from their contact with the surface and may therefore distort the measurements. Additionally, sensitivity also may be a problem, since electrically active contamination concentrations well below the resistivity measurement capability of the tool can cause performance degradation in the device under test.
Another popular method of measuring the free charges in the semiconductor device is the use of secondary ion mass spectroscopy (SIMS). The SIMS technique bombards the surface of the device under test with high energy charged particles in a "sputtering" fashion. These ions penetrate into the device, to a depth that is a function of their energy level, and excite a secondary ion emission from the device that is proportional to a contamination concentration level. The SIMS then measures the type and concentration of this free charge contamination. However, SIMS suffers from the severe limitation in that it measures concentration levels down to only about 5.0E14 atoms/ml for phosphorous (n-type). Therefore, contaminant concentrations below this level in semiconductor devices are not detected and may still cause serious performance problems. Additionally, the time required for the SIMS measurement process is a function of the target concentration level and may take days to determine the lower levels of contamination.
In summary, these measurement techniques typically require long periods of time to apply and do not have the measurement sensitivity to detect low contaminant concentration levels. Therefore, the information obtained from these tests is not available or detectable at the desired time during the fabrication process. This generally forces the testing to be done after the device has been fully fabricated and the majority of the manufacturing costs have been incurred. Moreover, it is not assured that trace amounts of contaminants will be detected. Additionally, since testing is performed on completed devices, as contrasted with devices that are still in the fabrication process, it is often difficult to determine the exact source or location of the contamination.
Accordingly, what is needed in the art is a way to quickly measure low levels of electrically active contaminants within the semiconductor device.